(1) Field of the Invention
This invention relates to improvements of a high-pressure tank into which high-pressure gas is charged and improvements of a method for fabricating the same.
(2) Description of the Related Art
In high-pressure tanks into which gas, such as natural gas or hydrogen gas, is charged and stored at high pressure, stress is concentrated at a gas discharge section and a dome section continuous therewith. Therefore, when the dome section and the gas discharge section protruding from the dome section are formed by necking a predetermined region of an elongated hollow cylindrical blank continuing from its opening end by means of spinning, they are increased in thickness as compared with the cylindrical section of the tank.
In this connection, the applicant filed a patent application on a method for forming a dome section and a gas discharge section by flow forming the predetermined region of the elongated hollow cylindrical blank continuing from its opening end into a larger thickness than the other region and then necking the thickened predetermined region through spinning, and has already obtained a patent for the method (see Japanese Patent Publication No. 3251216).
For the conventional general fabrication method described above, however, there is a limit to how much the thickness of the gas discharge section and the dome section can be increased because of the use of an elongated hollow cylindrical blank having a uniform thickness as a whole. For example, as shown in FIG. 3 as Comparative Example 1, with a tank obtained using an elongated hollow cylindrical blank 111 having a cylindrical section about 3.3 mm thick, even a gas discharge section 5, which is the thickest of all tank parts, has a thickness of only about 7.1 mm or about twice that of the cylindrical section 2. The resultant tank is difficult to adapt as a high-pressure tank 1 capable of withstanding further increased high pressures that would be expected in future.
To cope with this, as shown in FIG. 4 as Comparative Example 2, it can be considered to ensure the thickness of a gas discharge section 5 and a dome section 4 by increasing the thickness of an entire cylindrical section of an elongated hollow cylindrical blank 111 to about 7.6 mm, for example (in this case, the thickness of the gas discharge section 5 will be about 14.2 mm). In this case, however, the entire high-pressure tank 1 thus fabricated will also be thickened and thereby increased in weight, which hinders weight reduction. In addition, since the tank uses the thick elongated hollow cylindrical blank 111, its material cost will be high.
On the other hand, for the first-mentioned example disclosed in the Publication, since the predetermined region of the elongated hollow cylindrical blank continuing from the opening end is increased in thickness as compared with the other region, the portions which will serve as the gas discharge section and the dome section can have a predetermined thickness with certainty and the entire tank weight can be reduced by thinning the cylindrical section. In this example, however, since the portions which will serve as the gas discharge section and the dome section have a uniform thickness as a whole, the amount of reduction of the blank will be large. This may invite the complete closure of the gas discharge section and thereby necessitate subsequent boring of a gas outlet in a later process step, which requires extra labor.
It should be noted that the references assigned to the respective elements in FIGS. 3 and 4 correspond to the references assigned to respective elements in an embodiment of the present invention.